1. Field of the Invention
The present invention relates to seat suspension systems, and more particularly to systems of the type which provide damped movement of the seat in a generally vertical direction to compensate for bumpy and uneven terrain negotiated by a vehicle in which the seat is mounted.
2. History of the Prior Art
It is known to provide seat suspension systems which allow for damped vertical movement of the seat in response to motion of the vehicle in which the seat is mounted. Such suspension systems which may also include apparatus for adjusting the height of the seat are common in off-road vehicles such as tractors where bumps and uneven terrain are commonly encountered and where operator comfort is therefore of considerable importance.
Certain types of seat suspension systems provide the seat with a fixed amount of vertical damping. The damping is vertically adjustable, but once adjusted remains the same despite the occurrence of different terrain and different ride conditions. Where the vehicle operator can see or anticipate relatively rough terrain, the damping control can be adjusted to provide a high level of damping. Such condition is desirable if not necessary when relatively rough terrain and large bumps are being negotiated, but at the same time provides a ride which is unduly harsh and uncomfortable. For this reason when relatively even terrain which is not too bumpy is encountered, the operator typically decreases the amount of damping to soften the vertical suspension of the seat and thereby greatly increase operator comfort.
The difficulty with a fixed damping suspension system lies in the fact that terrain can change abruptly. For example, a relatively smooth road or field being traversed by the vehicle may have one or more large, hidden bumps. If the suspension system is adjusted to provide a low damping, soft ride desired on relatively even and non-bumpy terrain, the sudden encounter of a large bump or series of bumps can cause the seat to undergo substantial vertical movement in response thereto and perhaps even "bottom out" by striking the lower limit of vertical travel thereof. These and other situations often dictate the desirability of a suspension system which provides damping that varies in response to such things as the frequency of seat motion. Many prior art seat suspension systems optimize seat damping at one particular frequency at the expense of far less than optimum ride conditions at other frequencies of seat motion.
Examples of prior art seat suspension systems are provided by U.S. Pat. Nos. 3,265,345 of Vuichard, 3,632,077 of Hall, 3,912,248 of Pickford et al, 3,486,417 of Di Vita et al, 3,599,956 of Harder et al, 3,712,603 of Hickinbotham, 3,470,692 of Kamp and 3,938,770 of Turner et al. The Vuichard patent suggests placing a unidirectional flow restricter of fixed size in the form of the parallel combination of a check valve and a fixed flow regulator between a cylinder which mounts the seat to permit vertical movement thereof and an accumulator, so as to restrict upward seat movement and thereby damp out seat oscillations. The Hall patent describes a similar system in which upward movement or rebound of the seat is slowed by slowing the rate of relaxation of the flow regulator within the suspension system using a restricter and check valve arrangement. The valve arrangement can be reversed to slow downward movement instead of upward movement as pointed out in the Hall disclosure. Hall gets varied damping using flow regulation which requires mechanical linkages in a rather complex arrangement. As in the case of Vuichard damping is directly dependent on the position of the seat rather than on the velocity or changing displacement of the seat.
Accordingly, it would be desirable to provide a seat suspension system in which variable damping is provided as a function of seat velocity or the rate of change of seat displacement. It would furthermore be desirable to provide such a system in which the increased damping is provided in the downward direction only of seat movement. It would furthermore be desirable to provide a system with these capabilities which at the same time is of relatively simple arrangement and which does not require mechanical linkages to accomplish flow regulation.